> Billy[SMTP:[EMAIL PROTECTED] > > > On Tue, Jul 08, 2003 at 01:26:46PM -0400, Trei, Peter wrote: > > While the ear can't hear above 22KHz, signal above that *can* > > effect the perceived sound, by heterodyne effects. For example, > > if you play a single tone of 28KHz, or a single tone of 30 KHz, > > you can't hear them. Play them together, however, and you > > *can* hear a beat frequency of 2KHz. > > Bullshit detector buzzing. > Is this *really* true? Have you tried it? > > The beat frequency is an amplitude envelope around the 30kHz tone > (think AM). No part of its spectrum falls within audible range. > It shouldn't be audible at all. > Not personally, but The Net holds all knowledge. People are making real products using this technique. For example....
Here's a neat application - 'audio spotlights'. The directionality of a speaker is a function of the ratio of its diameter to the wavelength of the sound produced - by using an ultrasonic speaker with audible beat frequencies, you can make a small, very directional speaker: http://www.bostonaes.org/archives/2003/Jan/ http://www.acoustics.org/press/133rd/2pea.html Peter Trei
